In the field of medical image technology such as Positron Emission Tomography (PET) or gamma cameras, often a remote detector unit separated from a control unit is used. In a PET scanner 100, as for example shown in FIG. 1, a plurality of scintillators 130 and associated photomultiplier tubes (PMT) 110 or avalanche photodiodes (APD) are usually arranged in a circle of a detector ring 130. Such a detector ring 130 surrounds the patient to be scanned. To conduct a so-called PET scan, a short-lived radioactive tracer isotope, which decays by emitting a positron, is injected usually into the blood circulation of a living subject. After the metabolically active molecule becomes concentrated in tissues of interest, the research subject or patient is placed in the imaging scanner. The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour.
As the radioisotope undergoes positron emission decay, it emits a positron, the antimatter counterpart of an electron. After traveling up to a few millimeters the positron encounters and annihilates with an electron, producing a pair of gamma photons moving in almost opposite directions. These are detected when they reach a scintillator material in the scanning device, creating a burst of light which is detected by photomultiplier tubes (PMT) or silicon avalanche photodiodes (Si APD). The technique depends on simultaneous or coincident detection of the pair of photons.
A PMT or APD can be used in many imaging systems, such as PET scanners and gamma cameras. Each PMT or APD produces one or more signals that need to be processed to generate an image from a plurality of single events that are detected by a PMT. Both, a PMT and an APD require generally a high bias voltage which has to be supplied by a central often remotely located control device. FIGS. 2 and 3 show different embodiments for exemplary conventional PMTs. According to FIG. 2, a positive high bias voltage, for example, between 500 to 3000 V, is supplied to a resistor network 240-249 through terminal 230. Resistor network 240-249 is coupled between the cathode K and anode A of the PMT. The nodes between the resistors 240-249 are coupled with a plurality of dynodes D1-D10. The output signal can be obtained at terminal 220 and is decoupled via a capacitor 210 from the high bias voltage. The resistor network is designed such that a proper voltage gradient is set up between the dynodes D1-D10. This voltage gradient can be adjusted by a potentiometer 260 which is, for example, coupled with dynode D7 as shown in FIG. 2. However, other adjustment methods of the voltage gradient are also possible.
FIG. 3 shows another embodiment using a negative high bias voltage applied between a resistor network 340-348 coupled between the cathode K and the last dynode D10 via terminal 320. Again a potentiometer 360 coupled to an intermediate dynode D7 is used to adjust the voltage gradient. In this embodiment, the output signal can be received directly from the anode A via terminal 310.
FIG. 4 shows a respective high voltage bias circuit 400 as used for an avalanche photo diode detectors. A resistor network 420, 430, 440 is designed to adjust a high voltage received at terminal 410 by means of for example a manual potentiometer 430. The adjusted voltage is fed to the avalanche photo diode 460 through another resistor 450. The output signal is decoupled from diode 460 by means of a capacitor 470 and fed to the input of an operational amplifier 495 comprising a feedback network 480, 490. The output signal is fed to a terminal 405.
In either embodiment, the high bias voltage needs to be provided by a remote control unit. Thus, a respective high voltage line is provided between the control unit and the detector ring 130. In conventional systems the high voltage bias is adjusted by conventional potentiometers as shown in FIGS. 2-4.
Thus, there exists a need for an improved system that allows for a remote control of the voltage gradient or a remote control of the bias of a PMT or APD.